A positioning device that uses an electrostatic capacitive distance sensing apparatus is a device capable of very precise positioning of the subject of positioning. The general structure of this positioning device is shown in FIG. 1. FIG. 1 is referred to below. A conventional positioning device 100 that uses an electrostatic capacitive distance sensing apparatus comprises a moveable apparatus 200 comprising a stationary part (not illustrated) and a moveable part (not illustrated); an electrostatic capacitive distance sensing apparatus 300; a drive apparatus 400 for driving this moveable part in order to move the subject of positioning; and a control part 500 for controlling drive apparatus 400 based on the actual distance of movement of this moveable part measured by electrostatic capacitive distance sensing apparatus 300. Moveable apparatus 200 comprises two electrodes constituting a capacitor for measuring the distance of movement of an object. This moveable part connects with the subject of positioning. Movement of this movable part changes the distance between these two electrodes and changes the electrostatic capacity of this capacitor. Changes in this electrostatic capacity are measured by electrostatic capacitive distance sensing apparatus 300.
Electrostatic capacity can be converted to voltage using a capacity-voltage conversion circuit. There are two types of capacity-voltage converters and they differ in terms of the voltage conversion method. These types of capacity-voltage conversion circuits are shown in FIG. 2 and FIG. 3.
Capacity-voltage conversion circuit 310 shown in FIG. 2 measures the voltage proportional to the electrostatic capacity of the capacitor and inversely proportional to the distance d1 between electrodes. A signal source 311 in FIG. 2 is connected to one terminal of a capacitor 240 with a coaxial cable 250 in between. When it is unknown, the output voltage of signal source 311 is measured by a voltmeter 315. The other terminal of capacitor 240 is connected to the inverted input terminal of an amplifier 312 with a coaxial cable 260 in between. Amplifier 312 is the device that amplifies A-times the potential of the noninverted input terminal versus the inverted input terminal and outputs that potential, and the voltage is output such that the potential difference between the noninverted input terminal and the inverted input terminal becomes zero. The noninverted input terminal of amplifier 312 is connected to the reference potential. A reference capacitor 313 is connected in between the inverted input terminal and the output terminal of amplifier 312 and current flowing to capacitor 240 is converted to voltage. In addition, the output voltage of amplifier 312 is measured by a voltmeter 314. Voltage V1 measured by voltmeter 314 is as in the following formula. V1=−(Cs1/Cr1)·E1. Cs1 here is the capacity of capacitor 240. Cr1 is the capacity of reference capacitor 313. E1 is the output voltage of signal source 311. Cs1 is inversely proportional to the distance d1 between electrodes and therefore, V1 is also inversely proportional to the distance d1 between electrodes. The letter A entered in amplifier 312 is the amplification factor of amplifier 312, and this amplification factor is extremely large at the measured frequency point or the measured frequency band. The inverted triangles in the figure show the reference potential of the circuit.
A capacity-voltage conversion circuit 320 in FIG. 3 measures the voltage inversely proportional to the electrostatic capacity of this capacitor and proportional to the distance d1 between electrodes. A signal source 321 in FIG. 3 is connected to the inverted input terminal of an amplifier 322 with a reference capacitor 323 in between. When it is unknown, the output voltage of signal source 321 is measured by a voltmeter 325. Amplifier 322 is the device that amplifies A-times the potential of the noninverted input terminal versus the inverted input terminal and outputs that potential, and the voltage is output such that the potential difference between the noninverted input terminal and the inverted input terminal becomes zero. The noninverted input terminal of amplifier 322 is connected to the reference potential. The output voltage of amplifier 322 is measured by a voltmeter 324. One terminal of a capacitor 240 is connected to the inverted input terminal of amplifier 322 with a coaxial cable 250 in between, and the other terminal is connected to the output terminal of amplifier 322 with a coaxial cable 260 in between. Voltage V2 measured by voltmeter 324 is as in the following formula. V2=−(Cr2/Cs1)·E2. Cs1 here is the capacity of capacitor 240. Cr2 is the capacity of reference capacitor 323. E2 is the output voltage of signal source 321. Cs1 is inversely proportional to the distance between electrodes d1 and therefore, V2 is also proportional to the distance between electrodes d1. The letter A entered in amplifier 322 is the amplification factor of amplifier 322, and this amplification factor is extremely large at the measured frequency point or the measured frequency band. The inverted triangles in the figure show the reference potential of the circuit.
Electrostatic capacitive distance sensing apparatus 300 is capable of measuring the actual movement distance of this moveable part when it comprises either capacity-voltage conversion circuit 310 or capacity-voltage conversion circuit 320. Inverse operations are not necessary with capacity-voltage conversion circuit 320 and therefore, it is a convenient electrostatic capacitive distance sensing apparatus.
Capacity-voltage conversion circuit 310 and capacity-voltage conversion circuit 320 both require that capacitor 240 is insulated from the reference potential. Moveable apparatus 200 comprises two electrodes that constitute capacitor 240. Consequently, moveable apparatus 200 comprises two electrodes insulated from the reference potential. Moveable apparatus 200 will be described here while referring to FIGS. 4 through 7. FIG. 4 is an oblique view of moveable apparatus 200. Moveable apparatus 200 comprises a stationary part 210 and a moveable part 220. Moveable part 220 can move in the direction shown by arrow D1. Stationary part 210 and moveable part 220 are connected by a spring 230. Moreover, capacitor 240 is formed between a face 211 of stationary part 210 and a face 221 of moveable part 220. Coaxial cable 250 and coaxial cable 260 are connected to capacitor 240.
Next, FIG. 5 is face 211 of stationary part 210 seen from the front. FIG. 6 is face 221 of moveable part 220 seen from the front. In FIG. 5, face 211 comprises an electrode 241 with an insulator 242 in between. Moreover, in FIG. 6, face 221 comprises an electrode 243 with an insulator 244 in between.
Next, the A–A′ cross section of FIG. 4 is shown in FIG. 7. The structural elements in FIG. 7 that are the same as in FIGS. 4, 5, or 6 are shown by the same numbers and a detailed description thereof has been omitted. Moveable apparatus 200 in FIG. 7 comprises drive apparatus 400. Moveable apparatus 220 is driven by drive apparatus 400 and is capable of moving in the direction shown by arrow D1. As is clear from FIGS. 5 through 7, electrodes 241 and 243 constituting capacitor 240 are insulated from the reference potential.
The present inventors have discovered that electrode 241 and electrode 243 constituting capacitor 240 must be as close to one another as possible in order to very accurately measure any minute displacement of the moveable part of movable apparatus 200. This is because the S/N ratio of electrostatic capacitive distance sensing apparatus 300 decreases with a reduction in capacity Cs1 of capacitor 240. In terms of the S/N ratio, it is preferred that capacity Cs1 is at least several pF. Moreover, if electrode 241 and electrode 243 are close to one another, there is also an advantage in that changes in capacity Cs1 can be easily monitored. For instance, when the range of movement of moveable part 220 is 20 micrometers and electrode 241 and electrode 243 are disc-shaped electrodes with a diameter of 5 millimeters, electrode 241 and electrode 243 should face one another at a distance between electrodes of several micrometers to 10 micrometers. Electrode 241 and electrode 243 supported by insulators are not easily positioned parallel to one other with such a small distance in between. Moreover, even if it is flexible, coaxial cable 260 is a factor that interferes dynamically with the movement of moveable part 220. Furthermore, the force applied by coaxial cable 260 to moveable part 220 when coaxial cable 260 bends is not reproducible. Therefore, there is a problem with conventional positioning devices in that positioning errors are large.
The present invention provides a positioning device with a smaller positioning error than conventional devices. Additionally, the present invention provides a moveable apparatus with which dimensional uncertainty is reduced by making it possible to ground one of the two electrodes and eliminating the insulators that support the electrodes. The present invention also provides a distance sensing apparatus suitable for this type of moveable apparatus. Furthermore, the present invention reduces the cost of the positioning device.